1. Field of the Invention
The present invention relates to a fundus photographing apparatus, such as a fundus camera, used in an opthalmological clinic.
2. Description of the Related Art
Conventionally, fundus photographing by a fundus camera has been widely used to perform screening in a medical check in a group or diagnose opthalmologic diseases. A photographed fundus image is generally recorded as digital data. The photographed data is stored in a mobile recording medium or a hard disk drive built in a personal computer (PC).
The conventional fundus photographing by a fundus camera includes fluorescent photographing performed by intravenously administering a fluorescent contrast agent into a subject, in addition to color photographing. However, with recent increase of the number of patients suffering from age-related macular degeneration (AMD), fundus auto-fluorescence (FAF) photographing draws attention as a new photographing method. Although the FAF photographing has been mounted on a laser scanning type optometry device in early days, the FAF photographing is beginning to be applied to a fundus camera in recent years.
The FAF photographing is different from the conventional fluorescent photographing, and is performed without intravenous injection of the fluorescent contrast agent into a subject. It has been known that lipofuscin, which is a fluorescent substance, is deposited on a retinal pigment epithelium of the AMD patient. The FAF photographing photographs fluorescence emitted by the lipofuscin, and finds out a lesion characteristic of the AMD.
However, the fluorescence emitted by lipofuscin on a fundus retina is extremely weak, and it is difficult for a conventional photographing film or a digital image sensor to obtain an FAF image which can be used for a clinical examination by only one photographing. Therefore, there is a method for executing FAF photographing a plurality of times and performing addition combination of the obtained plurality of FAF images to obtain an FAF image having a better image quality.
When a fundus photographing apparatus performs addition combination of a plurality of fundus images, slight position deviations may occur between each fundus image due to insufficient fixation or micro-motion of fixation of a subject. Accordingly, addition combination could not be performed preferably. Therefore, Japanese Patent Application Laid-Open No. 2007-252707 discusses that it is indispensable for the addition combination of fundus images to combine the images after adjusting position deviations occurring between the images.
As described above, since fluorescence on the fundus retina of the AMD patient is extremely weak, it is necessary to greatly increase a sensitivity of an image sensor when the FAF photographing is performed. However, an FAF image acquired by photographing with increased sensitivity includes a lot of noise components. Thus, a primary FAF image may be buried in the mass noise components.
A conventional method which extracts a characteristic high-frequency component from an image could misunderstand the noise component as a characteristic area, so that a position deviation may not be properly corrected. Further, searching a characteristic area from an entire image is processing including a heavy load in itself, and the processing needs much time for correcting the position deviations.